Solvent borne polyurethane process

ABSTRACT

A process for obtaining a solvent borne polyurethane composition comprising preparing an isocyanate-terminated prepolymer A;
     reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0.1 to 1.8 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound and or chain-extending compound;   optionally introducing an isocyanate-terminated compound B; and   reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in the previous steps and isocyanate-terminated compound B with 0 to 1.2 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound and or chain-extending compound.

This application is a continuation of commonly owned co-pending U.S. application Ser. No. 11/994,664, filed Jan. 4, 2008, which in turn is the national phase application under 35 USC §371 of PCT/EP2006/006900, filed Jul. 14, 2006, which designated the U.S. and claims priority to European Patent Application No. 05381037.0, filed Jul. 14, 2005, the entire contents of each of which are hereby incorporated by reference.

The invention relates to processes for preparing a solvent borne polyurethane composition, the resultant solvent borne polyurethane composition, and an ink, in particular a printing ink comprising the solvent borne polyurethane composition for use in a laminate.

Laminates are multi-layered composites where each layer consists of the same or different materials. In the field of flexible packaging, laminates usually comprise plastic and/or metallised films. Flexible packaging is used for example in the food industry and has many requirements such as for example eliminating or limiting the transfer of moisture, oxygen, oils and flavours; flexible packaging used for microwave cooking needs to protect the contents during storage but also needs to have good heat resistance; flexible packaging used for beverages needs to have good cold resistance and handling resistance; flexible packaging used in other applications may also need to be resistant to the transfer to perfumes, resistant to surfactants, resistant to oil/water mixtures, and additionally the flexible packaging should be easy to open when required.

Generally laminates are produced by joining two or more layers using adhesives or by carrying out adhesive-free extrusion coating. Additionally it is often desirable to apply an image to one or more of the layers during the lamination process.

For example, if using an adhesive laminating method an image may be printed onto a plastic film substrate, after which an adhesive is applied to the inked substrate, followed by applying a second film to the adhesive (the adhesive could also be applied to the second film). If using an extrusion coating/laminating method an image may be printed onto a plastic film substrate, optionally followed by the application of a primer and then a molten resin is extruded onto the inked substrate to form a second layer followed by the formation of a bond between the two substrates. It is therefore desirable that laminating inks possess excellent adhesion to the printing substrate as well as to the film adhesive and/or film to be laminated.

Laminated films, when used to make packaging, often undergo heat sealing and when used as food packaging must be able to undergo a boiling or retorting treatment for cooking or sterilising the contents. It is therefore also desirable that delamination does not occur during such processes.

The properties of a laminate therefore depend on the type of films used, the laminating process, the type of adhesive and the ink properties and in particular the properties of any resins used as binders in the ink.

The types of films that are used in flexible packaging laminates include, among others polyester, cellophane, polypropylene, polyethylene, aluminium foils, nylon and paper. Such films may also have been functionalised through a range of chemical and physical treatments.

A range of binders have been used in laminating inks such as modified PVC (polyvinyl chloride), polyvinyl butyral, polyamides, polyesters, nitrocellulose and polyurethanes. However it has been found that some binders are incompatible, difficult to clean up from ink printing equipment and many only adhere to certain substrates and even if the binders do adhere, they may be poor in their resistance to boiling or retorting treatments and generally do not achieve desirable bond strength. In addition there is an increase in demand for high line speed printing, especially at line speeds greater than 200 m/min or even greater than 300 m/min. However, at such line speeds printability problems such as cob-webbing may occur for flexo printing and scumming may appear for gravure printing processes. One cause of such problems is the limited resolubility, which is sometimes also described as the redispersibility of the binders used in the inks in the typical solvents used in these applications. Resolubilty or redispersibility is a property, well known to the printing industry, whereby dry or drying polymer obtained from a polymer composition is redispersible or redissolvable in that same composition when the latter is applied thereto.

Although the use of solvents such as ketones or solvents with a slower evaporation rate may be used to solve some of the problems, these solvents may have other inherent issues such as safety and environmental issues as well as that they may be slow to dry which can result in the solvent migrating into the packed material. Therefore the use of solvents with a faster evaporation rate is useful, although if they are too fast then printability failures may also occur.

There are also concerns with undesirable chlorine containing compounds (notably hydrochloric acid and phosgene) being given of during the incineration of packaging printed with polyvinyl chloride based inks and polyvinyl butyral based inks.

A method for overcoming such problems is to use a combination of binders where binders may be chosen to suit particular films and adhesives. However a disadvantage with such an approach is that a large number of binders need to be prepared and stored for all the different types of inks that may be applied to the laminate films. Additionally if the various inks are not compatible with each other then extensive cleaning of the printing equipment would be required for each change over.

Traditional low molecular weight polyurethanes may give good printability but only have a limited application for laminates and usually require blending with at least a second harder binder to achieve a good balance in properties.

Elastomeric higher molecular weight polyurethanes are widely used in laminating and can give a good balance in properties however they have reduced printability especially in high speed printing processes.

EP 604890 B1 discloses a printing ink composition for a laminate comprising a polyurethane resin where the polyurethane is prepared with a low and a high molecular weight polyol. WO 02/38643 discloses solvent based poly(urethane/urea) resins suitable for laminating printing inks where the polyurethane prepolymer is derived from a blend of a polymeric diol and a diol. WO 01/14442 discloses a polyurethane resin obtained by preparing an isocyanate-terminated prepolymer which is then reacted with a diamine which suitable for formulating printing ink compositions. EP 1229090 A1 discloses a polyurethane resin, soluble in organic solvents, where the polyurethane is prepared with at least three polyols within different molecular weight ranges and where the polyurethane resin can be used in a printing ink for making laminates.

A disadvantage of such polyurethanes is that they often still require combining with other binders to get a good balance of properties such as for example adhesion, block resistance, flexibility and heat resistance. Furthermore the prior art does not describe a polyurethane binder that provides a good bond strength whilst maintaining printability at high line speeds.

Surprisingly we have found that it is possible to prepare polyurethane binders that overcome many of the disadvantages of the prior art systems with a polyurethane system which is suitable for inter alia flexo and gravure printing processes on a broad range of substrates used in flexible packaging film laminates and which are suitable for extrusion lamination.

According to the present invention there is provided a process for obtaining a solvent borne polyurethane composition comprising the following steps:

-   -   1) preparing an isocyanate-terminated prepolymer A;     -   2a) reacting the isocyanate groups of isocyanate-terminated         prepolymer A with 0 to 0.95 stoichiometric equivalents of at         least one active-hydrogen containing chain-terminating compound;     -   2b) reacting the isocyanate groups of isocyanate-terminated         prepolymer A with 0 to 0.95 stoichiometric equivalents of at         least one active-hydrogen containing chain-extending compound;         -   where steps 2a) and 2b) together are in the range of from             0.1 to 1.8 stoichiometric equivalents;     -   3) optionally introducing an isocyanate-terminated compound B;     -   4a) reacting the isocyanate groups of the isocyanate-terminated         prepolymer obtained in steps 2a) and 2b) and isocyanate groups         of the isocyanate-terminated compound B introduced in step 3)         with 0 to 1.0 stoichiometric equivalents of at least one         active-hydrogen containing chain-terminating compound;     -   4b) reacting the isocyanate groups of the isocyanate-terminated         prepolymer obtained in steps 2a) and 2b) and the isocyanate         groups of the isocyanate-terminated compound B introduced in         step 3) with 0 to 1.2 stoichiometric equivalents of at least one         active-hydrogen containing chain-extending compound;         -   where steps 4a) and 4b) together are in the range of from 0             to 1.2 stoichiometric equivalents;         -   where the process comprises at least one step with an             active-hydrogen containing chain-extending compound and at             least one step with an active-hydrogen containing             chain-terminating compound;         -   where if 0 wt % of the isocyanate-terminated compound B is             introduced then step 2a) comprises 0.1 to 0.95             stoichiometric equivalents of at least one active-hydrogen             containing chain-terminating compound and at least a step             2a) is performed before a step 2b).

Preferably the isocyanate-terminated prepolymer A in step 1 has an isocyanate group content in the range of from 0.5 to 5.5 wt %, more preferably 0.5 to 4.5 wt % and most preferably 0.5 to 3.5 wt %.

Preferably, if necessary, the final steps are repeated until the isocyanate group content in the composition is 1 wt % more preferably 0.5 wt % and more preferably 0.01 wt % and is most preferably so low that the isocyanate group content is no longer detectable using methods well known in the art.

Step 2a) and step 2b) of the process of the invention may be in any order. Step 4a) and step 4b) of the process of the invention may be in any order. Furthermore step 2a) and 2b), step 3), step 4a) and step 4b) may be repeated.

Preferably step 2a) comprises reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.8, more preferably 0.1 to 0.7 and most preferably 0.1 to 0.6 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound.

Preferably step 2b) comprises reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95, more preferably 0 to 0.9 and most preferably 0 to 0.8 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound.

Preferably steps 2a) and 2b) together are in the range of from 0.2 to 0.95, more preferably 0.2 to 0.9, most preferably 0.2 to 0.85 and especially 0.3 to 0.85 stoichiometric equivalents.

Preferably step 3) comprises introducing 0 to 1600%, more preferably 0 to 1000%, even more preferably 0 to 500%, especially 0 to 150% and most especially 0 to 100% by weight of isocyanate-terminated prepolymer A of an isocyanate-terminated compound B.

Preferably step 4a) comprises reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and the isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 0.55, more preferably 0 to 0.45 and most preferably 0 to 0.42 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound.

Preferably step 4b) comprises reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and the isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.1, more preferably 0 to 0.95 and most preferably 0 to 0.85 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound.

Preferably steps 4a) and 4b) together are in the range of from 0.3 to 1.2, more preferably 0.3 to 1.1 and most preferably 0.4 to 1.1 stoichiometric equivalents.

Preferably if 0 wt % of the isocyanate-terminated compound B is introduced then step 2a) comprises 0.1 to 0.55, more preferably 0.1 to 0.45 and most preferably 0.1 to 0.42 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound.

As is clear to those skilled in the art more than 1 stoichiometric equivalents of a chain-terminating compound or more than 1 stoichiometric equivalents of a chain-extending compound or more than 1 stoichiometric equivalents of a chain-terminating and a chain-extending compound together, based on the amount of free-isocyanate groups available would not react with any isocyanate groups and would be present as an excess. Where 1 or more than 1 stoichiometric equivalents is used in the present invention this also includes having an excess of chain-terminating and chain-extending compounds present. For example if ethanol is used as a solvent, it can under the right reaction conditions such as elevated temperatures or prolonged reaction times act as a chain-terminating compound and yet it is present in an excess. Furthermore when an excess of chain-terminating and or chain-extending compounds are present these may advantageously react with any introduced isocyanate-terminated compound B.

By solvent based composition is meant that the liquid medium in the composition substantially comprises organic solvents. Preferably the resultant solvent based polyurethane composition comprises 80 to 25 wt %, more preferably 70 to 30 wt %, most preferably 60 to 35% and especially 50 to 40 wt % of liquid medium. Preferably the liquid medium comprises 20 wt % of water, more preferably ≦6 wt % and most preferably ≦1 wt % of water. Preferably the liquid medium comprises ≧75 wt %, more preferably ≧90 wt %, most preferably ≧98 wt % and especially 100 wt % of fast evaporating solvents. Fast evaporating solvents may be defined as solvents having a molecular weight ≦125 g/mol and more preferably 105 g/mol. More preferably fast evaporating solvents are defined as solvents having an evaporation rate of ≧1.0, more preferably ≧1.4 and preferably ≧1.6. Values for evaporation rates were published by Texaco Chemical Company in a bulletin Solvent Data: Solvent Properties (1990). (The values given are relative to the evaporation rate (ER) of butyl acetate which is defined as 1.00). Determination of evaporation rates of solvents that are not listed in the Texaco bulletin is as described in ASTM D3539. Fast evaporating solvents are particularly useful where fast drying times are required, especially when printing onto hydrophobic and non-absorbent substrates, for example plastics, metal and glass.

Examples of such solvents include alcohols (such as ethanol, isopropanol, n-butanol, n-propanol), esters (such as ethyl acetate, propyl acetate), aromatic solvents (such as toluene), ketone solvents (such as acetone, methyl ethyl ketone, methyl isobutyl ketone) cyclohexanone, diacetone alcohol; aliphatic hydrocarbons; chlorinated hydrocarbons (such as CH₂Cl₂); ethers (such as diethyl ether, tetrahydrofuran) and mixtures thereof. Most preferably the liquid medium comprises a solvent selected from the group comprising ethanol, isopropanol, ethylacetate and or a mixture thereof. The liquid medium may also comprise other organic solvents such as ethoxy propanol and propylene glycol n-propyl ether.

The resultant polyurethane composition as obtained by the process of the invention also preferably has a viscosity ≦18,000 mPa·s, more preferably ≦12,000 mPa·s, most preferably ≦10,000 mPa·s and especially ≦5,000 mPa·s at any solids content in the range of from 20 to 75 wt %, more preferably 35 to 75 wt %, most preferably 45 to 75 wt % and especially 50 to 75 wt % in a solvent comprising ≧70 wt %, more preferably ≧90 wt % and most preferably 100 wt % of at least one solvent having an evaporation rate of ≧1.0. All viscosities are measured according to ISO 2555-1989 at 25° C. Preferred solvents used to measure the viscosity of the polyurethane in, include ethanol, isopropanol, n-propanol, ethyl acetate, propyl acetate and or mixtures thereof.

The isocyanate-terminated prepolymer A is preferably obtained by reacting components comprising (i) at least one polyisocyanate; (ii) optionally at least one isocyanate-reactive polyol with a weight-average molecular weight in the range of from 50 to 200 g/mol; (iii) at least one isocyanate-reactive polyol with a weight-average molecular weight in the range of from 201 to 20,000 g/mol; and optionally (iv) an isocyanate-reactive polyol not comprised by (ii) or (iii); optionally in the presence of a liquid medium and optionally in the presence of a catalyst. The isocyanate-terminated prepolymer A may comprise one or more than one isocyanate-terminated prepolymer A.

The isocyanate-terminated compound B may be a polyisocyanate as described below (for convenience called polyisocyanate B). The isocyanate-terminated compound B may also be an isocyanate-terminated prepolymer. If the isocyanate-terminated compound B is an isocyanate-terminated prepolymer (for convenience described as isocyanate-terminated prepolymer B) it is preferably obtained by reacting components as described above for isocyanate-terminated prepolymer A. The isocyanate-terminated compound B may comprise one or more than one isocyanate-terminated prepolymer B and/or polyisocyanate B. If isocyanate-terminated compound B is an isocyanate-terminated prepolymer B, it may also be partially chain-terminated and or chain-extended before being introduced in step 3.

The polyisocyanate component (i) may be an aliphatic polyisocyanate, an aromatic isocyanate or mixtures thereof.

The term aromatic polyisocyanate (for the sake of clarity) is intended to mean compounds in which all the isocyanate groups are directly bonded to an aromatic group, irrespective of whether aliphatic groups are also present. Examples of suitable aromatic polyisocyanates include but are not limited to p-xylylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4′-methylene bis(phenyl isocyanate), polymethylene polyphenyl polyisocyanates, 2,4′-methylene bis(phenyl isocyanate) and 1,5-naphthylene diisocyanate. Preferred aromatic isocyanates include 2,4′-methylene bis(phenyl isocyanate) and 4,4′-methylene bis(phenyl isocyanate). Aromatic polyisocyanates provide chemical resistance and toughness but may yellow on exposure to UV light.

The term aliphatic polyisocyanate (for the sake of clarity) is intended to mean compounds in which all the isocyanate groups are directly bonded to aliphatic or cycloaliphatic groups, irrespective of whether aromatic groups are also present.

Examples include but are not limited to ethylene diisocyanate, para-tetra methylxylene diisocyanate (p-TMXDI), meta-tetra methylxylene diisocyanate (m-TMXDI), 1,6-hexamethylene diisocyanate, isophorone diisocyanate (IPDI), cyclohexane-1,4-diisocyanate and 4,4′-dicyclohexylmethane diisocyanate. Aliphatic polyisocyanates improve hydrolytic stability, resist UV degradation and do not yellow. Preferred aliphatic isocyanates include isophorone diisocyanate, 4,4′-dicyclohexylmethane diisocyanate and 1,6-hexamethylene diisocyanate.

Preferably at least 70 wt %, more preferably at least 85 wt % and most preferably at least 95 wt % of the polyisocyanate in component (i) has two isocyanate groups.

Aromatic or aliphatic polyisocyanates which have been modified by the introduction of, for example, urethane, allophanate, urea, biuret, uretonimine and urethdione or isocyanurate residues may be used for component (i).

Preferably isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B comprise 2 to 50 wt %, more preferably 5 to 45 wt % and most preferably 8 to 45 wt % of component (i).

The isocyanate-reactive components (ii) to (iv) will normally consist of a polyol component bearing isocyanate-reactive groups which may also bear other reactive groups. By a polyol component it is also meant to include compounds with one or more isocyanate-reactive groups such as —OH, —CHR¹—COOH where R¹ can be H, alkyl (more preferably C₁ to C₈ alkyl); —SH, —NH— and —NH₂.

Examples of component (ii) include but are not limited to 1,4-cyclohexyldimethanol, ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, 1,6-hexanediol, furan dimethanol, cyclohexane dimethanol, glycerol, trimethylolpropan, dimethylol propanoic acid (DMPA) and dimethylol butanoic acid (DMBA).

Preferably component (ii) has an average of 1.8 to 3 isocyanate-reactive groups and more preferably component (ii) has an average of 1.8 to 2.5 isocyanate-reactive groups. For example component (iii) may comprise a mixture of a triol and a diol which together have an average of 1.8 to 2.5 isocyanate-reactive groups.

Preferably the weight average molecular weight of component (ii) is in the range of from 62 to 200 g/mol and more preferably 84 to 200 g/mol.

Preferably isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B comprise 0 to 20 wt %, more preferably 0 to 10 wt % and most preferably 0 to 5 wt % of component (ii).

Examples of component (iii) and (iv) include but are not limited to polyols such as polypropylene glycols, poly(propylene oxide/ethylene oxide) copolymers, polytetrahydrofuran, polybutadiene, hydrogenated polybutadiene, polysiloxane, polyamide polyesters, isocyanate-reactive polyoxyethylene compounds, polyester, polyether, polyether ester, polycaprolactone, polythioether, polycarbonate, polyethercarbonate, polyacetal and polyolefin polyols. Generally polyester polyols provide good weathering, good adhesion, improved chemical resistance and toughness; polyether polyols provide good flexibility, elasticity and storage stability; polycaprolactone polyols provide improved weathering and better heat resistance than polyether polyols and better water resistance than adipate polyester polyols.

Polyester amides may be obtained by the inclusion of amino-alcohols such as ethanolamine in polyesterification mixtures. Polyesters which incorporate carboxy groups may be used, for example polyesters where DMPA and/or DMBA are used during the synthesis.

Polyether polyols which may be used include products obtained by the polymerisation of a cyclic oxide, for example ethylene oxide, propylene oxide or tetrahydrofuran or by the addition of one or more such oxides to polyfunctional initiators, for example water, methylene glycol, ethylene glycol, propylene glycol, diethylene glycol, cyclohexane dimethanol, glycerol, trimethylopropane, pentaerythritol or Bisphenol A. Especially useful polyether polyols include polyoxypropylene diols and triols, poly (oxyethylene-oxypropylene) diols and triols obtained by the simultaneous or sequential addition of ethylene and propylene oxides to appropriate initiators and polytetramethylene ether glycols obtained by the polymerisation of tetrahydrofuran. Particularly preferred are polypropylene glycols.

Components (iii) and (iv) may also include crosslinking groups. Crosslinking groups are well known in the art and include groups which crosslink at ambient temperature (20±3° C.) or at elevated temperatures by a number of mechanisms including but not limited to Schiff base crosslinking (for example the reaction of carbonyl functional groups with carbonyl reactive amine and/or hydrazine functional groups); silane crosslinking (for example the reaction of alkoxy silane groups in the presence of water) and epoxy groups crosslinking with epoxy-reactive functional groups or isocyanate curing, where hydroxy or amine (primary or secondary) functional polyurethanes are combined with polyisocyanates. Usually the polyisocyanates are added shortly before application. Alternatively, blocked polyisocyanates are used which deblock at elevated temperature after application. Isocyanate crosslinking is most preferred, when crosslinking is applied during the application process.

Preferably the weight average molecular weight of component (iii) is in the range of from 300 to 11,000 g/mol, more preferably 350 to 6,000 g/mol and especially 350 to 5,000 g/mol.

Preferably isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B comprise 0 to 95 wt %, more preferably 30 to 95 wt % and most preferably 40 to 92 wt % of component (iii).

Preferably isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B comprise 0 to 95 wt %, more preferably 0 to 6 wt %, most preferably 0 to 3 wt % and especially 0 wt % of component (iv).

Steps 2a and 4a comprise the use of an active-hydrogen chain-terminating compound. Steps 2b and 4b comprise the use of an active-hydrogen chain-extending compound.

Examples of chain-terminating compounds include mono-alcohols, amino-alcohols, primary or secondary amines and mono-functional hydrazines as are well known in the art. Di- or poly-functional isocyanante-reactive compounds may be used as a chain-terminating compound if only one isocyanante-reactive group reacts under the given conditions. Examples of such difunctional compounds include ethanol amine and diethanolamine. The chain-terminating compound may also be a mono-functional isocyanate.

Examples of chain-extending compounds include amino-alcohols, primary or secondary diamines or polyamines such as ethylene diamine, propylene diamine and cyclic amines such as isophorone diamine and 4,4′-dicyclohexylmethane diamine; hydrazine and substituted hydrazines such as, for example, dimethyl hydrazine, 1,6-hexamethylene-bis-hydrazine, carbodihydrazine, hydrazides of dicarboxylic acids and sulphonic acids such as adipic acid dihydrazide, oxalic acid dihydrazide, isophthalic acid dihydrazide, hydrazides made by reacting lactones with hydrazine, bis-semi-carbazide, and bis-hydrazide carbonic esters of glycols; azines such as acetone azine, and or mixtures thereof. Another suitable class of chain-extending compounds are the so-called “Jeffamine” compounds with a functionality of 2 or 3 (available from Huntsman). These are PPO or PEO-based di or triamines, e.g. “Jeffamine” T403 and “Jeffamine” D-400. In a special embodiment where the prepolymer has isocyanate-reactive functional groups (such as hydroxyl groups) a chain-extending compound may also be a difunctional isocyanate.

The isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B are most preferably obtained by reacting components comprising:

-   -   (i) 8 to 45 wt % of at least one polyisocyanate;     -   (ii) 0 to 5 wt % of at least one isocyanate-reactive polyol with         a weight-average molecular weight in the range of from 50 to 200         g/mol;     -   (iii) 40 to 92 wt % of at least one isocyanate-reactive polyol         with a weight-average molecular weight in the range of from 201         to 20,000 g/mol; and     -   (iv) 0 to 3 wt % of an isocyanate-reactive polyol not comprised         by (ii) or (iii);         -   where (i), (ii), (iii) and (iv) add up to 100%;         -   optionally in the presence of a liquid medium.

The isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B may be prepared conventionally by reacting a stoichiometric excess of the organic polyisocyanate (component (i)) with the isocyanate-reactive compounds (components (ii), (iii) and (iv)) under substantially anhydrous conditions at a temperature between about 30° C. and about 130° C., more preferably about 45° C. to about 85° C. until reaction between the isocyanate groups and the isocyanate-reactive groups is substantially complete to form an isocyanate-terminated prepolymer. Preferably the reactants used for the isocyanate-terminated prepolymer are used in proportions corresponding to a ratio of isocyanate groups to isocyanate-reactive groups of from about 1.2:1 to about 2:1, more preferably 1.3:1 to about 2:1 and most preferably from about 1.45:1 to 2:1.

If desired, catalysts such as dibutyltin dilaurate and stannous octoate, zirconium or titanium based catalysts may be used to assist the polyurethane formation. Optionally no catalyst is added. The reaction between the components may be carried out in any order.

The reaction is usually carried out in the presence of an organic solvent to control the viscosity. Suitable organic solvents include but are not limited to acetone, tetrahydrofuran, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone and other solvents well known in the art.

Some of the isocyanate groups of the isocyanate-terminated prepolymer A are then reacted with a chain-extending and/or chain-terminating component which is preferably added in a liquid medium at a temperature between 20 to 55° C.

The introduction of isocyanate-terminated compound B may be by means of a simple blend where isocyanate-terminated compound B is added to the reactor containing isocyanate-terminated prepolymer A or isocyanate-terminated compound B may be prepared in the presence of the isocyanate-terminated prepolymer A.

The remaining isocyanate groups of the isocyanate-terminated prepolymer A and optional isocyanate-terminated prepolymer B are reacted with a chain-extending and/or chain-terminating component which is preferably added in a liquid medium at a temperature between 20 to 55° C.

Examples of suitable processes include but are not limited to the following processes:

-   -   Preparation of isocyanate-terminated prepolymer A (1), partial         chain-termination (2a) and partial or full chain-extension (4b).     -   Preparation of isocyanate-terminated prepolymer A (1), partial         chain-extension (2b), partial chain-termination (4a) and         chain-extension (4b).     -   Preparation of isocyanate-terminated prepolymer A (1), partial         chain-termination (2a), introducing isocyanate-terminated         compound B (3), partial or full chain-extension (4b) and         optionally partial or full chain-termination (4a).     -   Preparation of isocyanate-terminated prepolymer A (1), partial         chain-extension (2b), introducing isocyanate-terminated compound         B (3), partial or full chain-termination (4a) and partial or         full chain-extension (4b).     -   Preparation of isocyanate-terminated prepolymer A (1), partial         chain-termination (2a), partial chain-extension (2b),         introducing isocyanate-terminated compound B (3) and partial or         full chain-extension (4b) and/or partial or full         chain-termination (4a).

In a second embodiment of the present invention there is provided a process for obtaining a solvent borne polyurethane composition comprising the following steps:

-   -   1) preparing an isocyanate-terminated prepolymer A;     -   2a) reacting the isocyanate groups of isocyanate-terminated         prepolymer A with 0.1 to 0.6 stoichiometric equivalents of at         least one active-hydrogen containing chain-terminating compound;     -   2b) reacting the isocyanate groups of isocyanate-terminated         prepolymer A with 0 to 0.8 stoichiometric equivalents of at         least one active-hydrogen containing chain-extending compound;     -   where steps 2a) and 2b) together are in the range of from 0.3 to         0.85 stoichiometric equivalents;     -   3) introducing 0 to 100% by weight of isocyanate-terminated         prepolymer A of an isocyanate-terminated compound B;     -   4a) reacting the isocyanate groups of the isocyanate-terminated         prepolymer obtained in steps 2a and 2b and isocyanate-terminated         compound B introduced in step 3 with 0 to 0.42 stoichiometric         equivalents of at least one active-hydrogen containing         chain-terminating compound;     -   4b) reacting the isocyanate groups of the isocyanate-terminated         prepolymer obtained in steps 2a and 2b and the         isocyanate-terminated compound B introduced in step 3 with 0 to         0.85 stoichiometric equivalents of at least one active-hydrogen         containing chain-extending compound;         -   where steps 4a) and 4b) together are in the range of from             0.4 to 1.1 stoichiometric equivalents;         -   where the process comprises at least one step with an             active-hydrogen containing chain-extending compound and at             least one step with an active-hydrogen containing             chain-terminating compound;         -   where if 0 wt % of the isocyanate-terminated compound B is             introduced then step 2a) comprises 0.1 to 0.42             stoichiometric equivalents of at least one active-hydrogen             containing chain-terminating compound and at least a step             2a) is performed before a step 2b).

Preferably the resultant solvent borne composition has a broad molecular weight distribution. More preferably the resultant solvent borne composition has a polymodal molecular weight distribution.

The polydispersity index (PDi) is defined as the weight average molecular weight (Mw) divided by the number average molecular weight (Mn). The Mw and the Mn is usually determined using Gel Permeation Chromatography (GPC) with polystyrene as a standard and tetrahydrofuran as an eluent. The PDi is calculated on the total molecular weight of the polyurethanes A and B of the invention composition.

Preferably the PDi of the polyurethanes in the resultant composition of the invention is in the range of from 1.3 to 10, more preferably 2.8 to 6, most preferably 3.0 to 6 and especially 3.0 to 5.0.

The Mp is the molecular weight with the highest signal (i.e. the apex of the peak) in a chromatogram resulting from the measuring of the molecular weight of the invention composition using Gel Permeation Chromatography (GPC) with polystyrene as a standard and tetrahydrofuran as an eluent. The Mp is also known as the peak Mw. Mp values are discussed in Modern Size Exclusion Liquid Chromatography, W. W. Yau, J. K. Kirkland and D. D. Bly, John Wiley & Sons, USA, 1997.

Preferably the Mp of the polyurethanes in the resultant composition of the invention is in the range of from 6,000 to 60,000 g/mol, more preferably 20,000 to 60,000 g/mol, most preferably 25,000 to 60,000 g/mol, and especially 30,000 to 60,000 g/mol.

Preferably the composition of the invention comprises at least: 10 to 90 wt %, more preferably 50 to 90 wt % and most preferably 75 to 90 wt % of a polyurethane having an Mp in the range of from 6,000 to 60,000 g/mol, more preferably 25,000 to 60,000 g/mol and most preferably 32,000 to 60,000 g/mol.

In a further embodiment of the present invention the resultant composition of the invention comprises polyurethanes having an Mp in the range of from 6,000 to 60,000 g/mol and a PEA in the range of from 2.8 to 6.

Preferably the resultant composition of the process of the invention is film-forming.

By film-forming is meant that on removal of most or all of the liquid medium from the polyurethane composition a cohesive film is formed. Furthermore it has a minimum level of strength and is dry to touch i.e. it can be slightly tacky but the material will not transfer to when it is touched. A non-film forming polyurethane will even in the absence of any liquid medium remain very tacky and like a paste.

After obtaining the resultant solvent borne polyurethane composition some or all of the organic solvent may be removed or further organic solvent and or water may be added to give a polyurethane solution with a solids content in the range of from 10 to 90 wt %, preferably 35 to 75 wt %, more preferably 42 to 65 wt % and most preferably 49 to 60 wt %.

The resultant solvent borne polyurethane composition has a balance between properties such as good solubility in solvents, good adhesion, bond strength, block resistance, heat resistance and printability.

The resultant solvent borne polyurethane composition of the invention may be used directly or in combination with, for example, fillers, waxes, thickeners, co-resins and/or colorants.

The resultant solvent borne polyurethane composition of the invention may be used as a binder for packaging inks.

In an embodiment of the present invention there is provided an ink comprising a solvent borne polyurethane composition obtained from the process according to the invention, a colorant and optionally additional organic solvent.

The ink preferably has a viscosity in the range of from 30 to 1,000 mPa·s and more preferably 30 to 500 mPa·s at 20° C.

The ink viscosity may also be measured using a cup type viscometer where the time it takes a sample to flow out of the orifice of the sample container is measured in seconds. An example of such a viscometer is a Ford Cup viscometer and a Ford Cup 4 viscometer has an orifice of 4 mm. This cup is suitable for ink viscosities in the range of from approximately 30 to 375 mPa·s and such inks usually flow out in about 10 to 100 seconds.

The ink preferably has a viscosity in the range of from 10 to 100 seconds, more preferably 10 to 75 seconds and especially 10 to 60 seconds when measured using a Ford Cup 4.

Preferably the ink has a solids content in the range of from 10 to 80 wt %, more preferably 10 to 70 wt % and most preferably 15 to 60 wt %.

Preferably the ink comprises 9 to 79 wt %, more preferably 9 to 70 wt % and most preferably 9 to 60 wt % of the solvent borne polyurethane obtained from the process of the present invention.

Preferably the ink comprises 1 to 50 wt %, more preferably 3 to 40 wt % and most preferably 8 to 40 wt % of a colorant.

Colorants include dyes, pigments or mixtures thereof. The pigment may be any conventional organic or inorganic pigment such as titanium dioxide, carbon black or any coloured pigments well known in the art.

The dyes may be any conventional dyes selected from acid dyes, natural dyes, cationic or anionic direct dyes, basic dyes and reactive dyes.

Optionally the ink may also contain other ingredients used in inks, for example defoamers, anti-oxidants, corrosion inhibitors, bacteriocides and viscosity modifiers.

The ink may be used in a number of printing processes including flexographic and/or gravure printing processes.

In a further embodiment of the present invention there is provided a process for printing an image on a substrate comprising applying thereto an ink comprising a polyurethane composition obtained from the process according to the present invention.

Preferably the printing process is a flexographic and/or gravure printing process.

The invention will now be described by example only. CE denotes a comparative example. All parts and percentages are by weight unless specified otherwise.

Materials Used:

-   IPDI=isophorone diisocyanate -   MDI=4,4′-diphenylmethane diisocyanate -   PPG 4000=Acclaim 4200=polypropylene glycol, Mw 4000 (Bayer) -   1,3-BG=1,3-butyldiglycol -   IPDA=isophorone diamine -   n-BA=n-butyl amine -   EtOH=ethanol (99.8%) (solvent and terminator) (BP) -   EA=ethylacetate (solvent) (BP) -   PP-A=isocyanate terminated prepolymer A -   Solsperse 20000=dispersing agent (Noveon) -   Solsperse 12000=dispersing agent (Noveon) -   Finntitan RD15=white pigment (Kemira) -   Sunfast Blue 249-0435=blue pigment (Sun Chemical) -   NC-DLX 3.5=nitrocellulose binder (Wolff) -   NC A-400=nitrocellulose binder (65% IPA) (Wolff) -   NeoRez U-351=non reactive, elastomeric polyurethane binder, 40%     solids in a 33:66 EA:EtOH solvent mixture (DSM NeoResins BV) -   COPP=substrate, MB 400, co-extruded bioriented polypropylene (Mobil) -   PET (Corona)=substrate Mylar 400C, polyester, corona treated (Toray     Plastic) -   PET (PVDC)=substrate, 14F760, polyester, PVDC treated (Deprosa) -   PET (Chemical)=substrate, Mylar 813, polyester, chemically treated     (Toray Plastic) -   PE=substrate, Polyethylene (Nordenia) -   s.a.=stoichiometric equivalents of unreacted NCO

EXAMPLES 1 TO 3, COMPARATIVE EXAMPLES C1 To C3 Step 1

An isocyanate-terminated prepolymer A was prepared as follows: A four-necked reaction vessel with an agitator, a thermometer, a condenser and a gas introduction pipe was charged with melted polyisocyanate(s) (component (i)). Isocyanate-reactive polyols (components (ii), (iii) and (iv)) were added to the reaction vessel. The temperature was increased to 45° C. under nitrogen. Tin catalyst was added to the reaction vessel. The temperature was increased to 65° C. and held for 120 minutes. The resultant NCO % was measured. While cooling the reaction mixture to a temperature between 55 and 20° C., liquid medium was added to uniformly dissolve the reaction mixture. The resultant NCO % was measured again.

Step 2

An active-hydrogen chain terminator in a liquid medium was added to the isocyanate-terminated prepolymer obtained in step 1 to chain terminate part of the isocyanate-terminated prepolymer. The resultant NCO % was measured.

Step 3

An isocyanate-terminated compound B was added to the reaction mixture obtained in step 2.

If compound B comprises an isocyanate-terminated prepolymer B then isocyanate terminated prepolymer B is prepared as follows:

A four-necked reaction vessel with an agitator, a thermometer, a condenser and a gas introduction pipe was charged with melted polyisocyanate(s) (component i)), and the isocyanate-reactive polyols (components (ii), (iii) and (iv)), were added to the reaction vessel. The temperature was increased to 45° C. under nitrogen. Tin catalyst was added to the reaction vessel. The temperature was increased to 65° C. and held for 120 minutes. The resultant NCO % was measured. While cooling the reaction mixture to a temperature between 55 and 20° C., liquid medium was added to uniformly dissolve the reaction mixture. The resultant NCO 96 was measured again.

Step 4

At a temperature between 55 and 20° C., an active-hydrogen chain extender and or active-hydrogen chain terminator in a liquid medium was added to the reaction vessel to chain extend the prepolymer. The reaction was continued until the final NCO % was below the limit of detection (0.1%).

The components and quantities used are given below in Table 1. The resultant properties are given below in Table 2.

TABLE 1 Examples 1 2 3 Isocyanate terminated prepolymer A - step 1 (i) MDI (g) 85.1 89.7 137.3 (i) IPDI (g) — — 35.5 (ii) 1,3-BG (g) 9.2 8.6 14.7 (iii) PPG-4000 (g) 440.5 413.5 704.6 Tin Octoate or * DBTL (g) 0.07 0.07 *0.27 NCO % 1.8 2.3 2.9 EA (g) 119.1 111.7 — NCO % 1.4 1.9 2.9 Chain terminate - step 2 n-BA s.a./(g) 0.4/(6.8)  0.3/(6.5)  — EtOH s.a./(g) 0.2 (6.4) EA (g) 74.6 70.0 294.4 NCO % 0.7 1.2 1.6 Isocyanate terminated Compound B - step 3 Compound B (%) 66 56 — PP-A from step 1 (g) 488.7 402.2 — Chain Extend - step 4b IPDA s.a./(g) 0.8/(19.5) 0.8/(25.0) 0.91/(35.4) Chain terminate - step 4a n-BA s.a./(g) 0.15/(3.0)  0.15/(3.9)  — Liquid medium EtOH (g) 580 470 570 Final solvent system of the composition EtOH % 64 61 65 EA % 36 39 35 Comparative Examples C1 C2 [[C4]] C3 Isocyanate terminated prepolymer A - step 1 (i) MDI (g) 69.2 79.2 125.5 (i) IPDI (g) — — 32.5 (ii) 1,3-BG (g) 7.4 7.6 13.3 (iii) PPG-4000 (g) 358.4 364.9 644.1 Tin Octoate or * DBTL (g) 0.06 0.06 *0.24 NCO % 1.2 1.8 2.2 EA (g) 96.8 160.4 270 NCO % 1.2 1.8 2.2 Chain terminate - step 2 n-BA s.a. (g) — — — Isocyanate terminated Compound B - step 3 Compound B (%) 100 100 — PP-A from step 1 (g) 592.6 612.2 — Chain Extend - step 4b IPDA s.a./(g) 0.8/(26.3) 0.8/(35.5) 0.91/(44.4) Chain terminate - step 4a n-BA s.a./(g) 0.15/(4.1)  0.15/(5.6)  — Liquid medium EtOH (g) 580 531 675 Final Solvent system of the composition EtOH % 64 61 71 EA % 36 39 29

TABLE 2 Examples 1 2 3 C1 C2 C3 Solids % 50.0 52.5 52.3 53.9 53.1 48.81 Viscosity 3450 3100 1600 41000 13200 14000 mPa · s Mw 59850 45900 37300 98100 70400 62500 PDi 3.9 3.6 3.2 3.1 3.1 2.8 Mp 58150 37800 33000 92700 63400 61300

Molecular Weight Determination

Gel Permeation Chromatography analysis for the determination of polymer molecular weights were performed on a Waters HPLC equipment, including a pump (Waters 515 HPLC pump), an autoinjector (Waters 717 plus), and a column oven. The eluent was a BHT stabilized tetrahydrofuran GPC grade, purity >99.8% (THF). The injection volume was 150 μl. The flow rate was established at 1.0 cm³/min. Three new PLgel 5 μm MIXED-D (300×7.5 mm) from Polymer laboratories with a guard column (PLgel 5 μm Guard, 50×7.5 mm) were applied at a temperature of 40° C.

The detection was performed with a differential refractive index detector (Waters 2410) at 40° C. internal temperature. 25 mg of sample were dissolved in 4 cm³ of THF, and the mixtures were left undisturbed for at least 1 hour. Calibration was performed with 8 polystyrene standards (Polymer Laboratories, Easical PS-2), ranging from 580 to 377,400 g/mol prepared at a concentration of 0.10% w/v. The calculation was performed with Empower PRO 2002 software (Waters) with a third order calibration curve (R>0.99990), and using a flat and horizontal baseline considering species between 400,000 and 500 g/mol.

The molecular weights are expressed in g/mol in this application and it should be understood that this is a molecular weight measured against polystyrene standards and derived using universal calibration as described above.

White Ink

Ingredients 1 to 4 as shown in Table 3 below were mixed into a homogeneous mixture. Ingredient 5 was added to the mixture and dispersed using a Cowles Dissolver or the like until a slurry consistency was reached and the pigment particles were less than 10 μm in size (about 30 minutes). Ingredient 6 was added under agitation. The viscosity of the ink was adjusted with ethanol to a viscosity of 18 to 22 sec as measured by a Ford Cup 4 at 20° C.

TABLE 3 No Ingredient Ink 1 Ink 2 Ink 3 Ink 4 1 Ethanol 12.5 12.5 12.5 12.5 2 Ethyl Acetate 4.2 4.2 4.2 4.2 3 Solsperse 20000 0.5 0.5 0.5 0.5 4 NC-DLX 3.5 2.1 2.1 2.1 2.1 5 Finntitan RD15 30.0 30.0 30.0 30.0 6 NeoRez U-351 30.0 — — — 6 Example 1 — 24.0 — — 6 Example 2 — — 22.9 — 6 Example 3 21.4

Adhesion

A film of the white ink prepared as described in Table 3 above was cast onto each substrate (6 μm wet) and dried with a hairdryer for 5 seconds.

A self adhesive tape (10 cm, type 683 of 3M) was applied under uniform pressure onto a printed ink layer on a substrate immediately after drying of the layer and torn off the substrate immediately thereafter. The quantity of the print adhered to the tape was classified with a scale from 0 to 5, where 0 means more than 95% of the printed layer adhered to the tape, 1 means more than 50% of the layer adhered to the tape, 2 means less than 30%, of the printed layer adhered to the tape, 3 means less than 20% of the printed layer adhered to the tape, 4 means less than 10% of the printed layer adhered to the tape and 5 means less than 2% of the printed film adhered to the tape. The results are shown in Table 4 below.

TABLE 4 Ink 1 Ink 2 Ink 3 Ink 4 COPP 5 5 5 5 PET (PVdC) 5 5 5 5 PET (Corona) 5 5 5 5 PET (Chemical) 5 5 5 5

Bondstrengths

The technique for the production of the laminate is the adhesive based lamination technique. A film of the white ink prepared as described in Table 3 above was cast onto each substrate (6 μm wet) and dried with a hairdryer for 5 seconds. After 2 hours an adhesive Novacote 275/Ca12, 4 μm wet, 33.3% solids was applied onto the white ink layer and the second substrate was applied to the adhesive layer. The laminates were rolled with a 10 kg roller and stored for 2 days at room temperature. Then the bond strength was measured by pulling the substrates apart using dynamo meter JBA model 853.

The bond strength is expressed as a combination of number value and letters. The number value stands for grams needed to separate the laminate of a width of 15 mm and a dynamometer speed of 100 mm/min and is given in Newton/15 mm. The higher the value, the greater the bond strength. The letters indicate the kind of breakage with regards to the layer of the printing ink. The results are given in Table 5 below.

TABLE 5 Lamination Ink 1 Ink 2 Ink 3 Ink 4 COPP/COPP 4.7R 4.8R 4.7R 2.7P COPP/PE 5.0R 2.7R 4.0R 3.4P PET (Chemical)/COPP 2.3R 2.6P 3.0P 1.8P PET (Chemical)/PE 4.2R 3.0P 3.1P 1.8P PET (PvDC)/COPP 4.3R 4.4R 2.2R 3.3R PET (PvDC)/PE 3.0R 2.0R 1.6R 2.9R PET (Corona)/COPP 1.6T 1.5T 1.3T 1.6T PET (Corona)/PE 1.1T 0.9T 1.0T 1.4T T: Transfer of 100% of the ink layer (from the substrate to the counter substrate) P: Splitting of the ink layer (between the substrate and the counter substrate) R: Break/Tear of one of the two films of the laminate.

Block Resistance

A OPP film printed with an ink layer was overlapped onto an unprinted OPP film which had been subjected to a corona-discharging treatment so that the printed and therefore inked surface of the former was brought into contact with the surface of the latter. The resulting assembly was allowed to stand under a load of 3 kg/cm² at 40° C. for one day. The unprinted OPP film was peeled off from the printed OPP film and the degree to which the ink film was transferred to the unprinted surface was observed and given a scale of 1 to 4 and the results are shown in Table 6 below:

4 means the unprinted OPP film was smoothly peeled off and none of the ink film was transferred thereto.

3 means the unprinted OPP film was not smoothly peeled off but none of the ink film was transferred thereto.

2 means that less than 50% of the ink film was transferred.

1 means that 50 to 100% of the ink film was transferred.

TABLE 6 Ink 1 Ink 2 Ink 3 Ink/film 3 3 3

Heat Resistance

Heat resistance (also known as the thermo resistance) was tested with a heat-sealing machine, Otto Brugger HSG/ETK. An area of a printed substrate in contact with aluminium foil was placed between the heat seal jaws previously preset to a temperature of at least 120° C. A dwell time of 1 second at a pressure of 3 bars/cm² was applied. After cooling the print and the foil were pulled apart and examined for ink pick off and ink transfer on to the foil. If the sample passed the test, the temperature was increased step by step until failure occurred. For a pass there should be no ink transfer from the print to the foil. An acceptable temperature for a fail is 140° C. The results are shown below in Table 7.

TABLE 7 Ink 1 Ink 2 Ink 3 Ink 5 COPP 180° C. 160° C. 160° C. 180° C.

Cob-Webbing

Cob-webbing is one of the causes of printing problems. The polyurethane compostions of the examples 1 to 3 and comparative examples 1 to 3 were formulated into blue ink to show improvement in the cob-webbing tendency.

Blue Ink

Ingredients 1 to 5 as shown in Table 8 below were mixed into a homogeneous mixture. Ingredient 6 was added to the mixture and dispersed using a Cowles Dissolver or the like until a slurry consistency was reached and the pigment particles were reduced to a size less than 10 μm (about 30 minutes). A solution of ingredients 7, 8 and 9 were added under agitation. The viscosity of the ink was adjusted with ingredient 10 to a viscosity of 30 to 40 seconds as measured by a Ford Cup 4 at 20° C.

TABLE 8 No Ingredient Ink 5 Ink 6 Ink 7 Ink 8 Ink 9 Ink 10 1 Ethanol 25.9 25.9 25.9 25.9 25.9 25.9 2 Ethyl Acetate 8.1 8.1 8.1 8.1 8.1 8.1 3 Solsperse 20000 1.6 1.6 1.6 1.6 1.6 1.6 4 Solsperse 12000 0.6 0.6 0.6 0.6 0.6 0.6 5 NC A-400 (65% IPA) 3.9 3.9 3.9 3.9 3.9 3.9 6 Sunfast Blue 249-0435 12.0 12.0 12.0 12.0 12.0 12.0 7 Ethanol 15.2 16.3 16.2 16.8 16.5 14.7 8 Ethyl Acetate 5.9 5.9 5.9 5.9 5.9 5.9 9 Example 1 22.1 — — — — — 9 Example 2 — 21.0 — — — — 9 Example 3 — — 21.1 — — — 9 Comparative — — — 20.5 — — Example 1 9 Comparative — — — — 20.8 — Example 2 9 Comparative — — — — — 22.6 Example 3 10 Ethanol 4.7 4.7 4.7 4.7 4.7 4.7

The inks were adjusted with further ethanol to a viscosity of 18 to 20 sec (Ford Cup 4 at 20° C.).

Five drops of the blue ink composition were put onto clean polyethylene film. The drops were cast immediately with a flexographic hand roller which was rolled 5 times (up & down), under a spot suction. The cob-webbing tendency was observed during the application.

After five minutes, the appearance of the ink applied onto the polyethylene film was judged with respect the amount of ink which is removed from the substrate, indicating different evolution of tack among the versions. The scale used was 0 to 5 with 5 being the best result. The results are shown in Table 9 below.

TABLE 9 Ink 5 Ink 6 Ink 7 Ink 8 Ink 9 Ink 10 Cob-webbing 3 4 4 0 1 2 Appearance 3 5 4 0 1 2 It is apparent that the low viscosity of examples 1 to 3 (inks 5 to 7) when compared to comparative examples 1 to 3 (inks 8 to 10) at equivalent solids (Table 1) contribute to an improvement in cob-webbing and inter alia printing properties. 

1. A process for obtaining a solvent borne polyurethane composition comprising 80 to 25 wt % of a liquid medium, where the liquid medium comprises <20 wt % of water and where the PDi of the solvent borne polyurethane is in the range of from 2.8 to 6, the process comprising the following steps: 1) preparing an isocyanate-terminated prepolymer A; 2a) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound selected from the group consisting of mono-alcohols, amino-alcohols, primary or secondary amines and mono-functional hydrazines; 2b) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 2a) and 2b) together are in the range of from 0.1 to 1.8 stoichiometric equivalents; 3) optionally introducing an isocyanate-terminated compound B; 4a) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.0 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound selected from the group consisting of mono-alcohols, amino-alcohols, primary or secondary amines and mono-functional hydrazines; 4b) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and the isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.2 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 4a) and 4b) together are in the range of from 0 to 1.2 stoichiometric equivalents; where the process comprises at least one step with an active-hydrogen containing chain-extending compound and at least one step with an active-hydrogen containing chain-terminating compound; where if 0 wt % of the isocyanate-terminated compound B is introduced then step 2a) comprises 0.1 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound and at least a step 2a) is performed before a step 2b).
 2. A process for obtaining a solvent borne polyurethane composition comprising the following steps: 1) preparing an isocyanate-terminated prepolymer A; 2a) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound; 2b) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 2a) and 2b) together are in the range of from 0.1 to 1.8 stoichiometric equivalents; 3) optionally introducing an isocyanate-terminated compound B; 4a) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.0 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound; 4b) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and the isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.2 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 4a) and 4b) together are in the range of from 0 to 1.2 stoichiometric equivalents; where the process comprises at least one step with an active-hydrogen containing chain-extending compound and at least one step with an active-hydrogen containing chain-terminating compound.
 3. A process according to claim 1 where steps 2a) and 2b) together are in the range of from 0.2 to 0.95 stoichiometric equivalents.
 4. A process according to claim 1 where step 3 comprises introducing 0 to 1600% by weight of isocyanate-terminated prepolymer A of an isocyanate-terminated compound B.
 5. A process according to claim 1 where the Mp of the solvent borne polyurethane is in the range of from 6,000 to 60,000 g/mol.
 6. A process according to claim 2 where the PDi of the solvent borne polyurethane is in the range of from 1.3 to
 10. 7. A process according to claim 1 where the Mp of the solvent borne polyurethane is in the range of from 6,000 to 60,000 g/mol and the PDi is in the range of from 2.8 to
 6. 8. A process according to claim 1 where isocyanate-terminated prepolymer A is obtained by reacting components comprising: (i) 8 to 45 wt % of at least one polyisocyanate; (ii) 0 to 5 wt % of at least one isocyanate-reactive polyol with a weight-average molecular weight in the range of from 50 to 200 g/mol; (iii) 40 to 92 wt % of at least one isocyanate-reactive polyol with a weight-average molecular weight in the range of from 201 to 20,000 g/mol; and (iv) 0 to 3 wt % of an isocyanate-reactive polyol not comprised by (ii) or (iii); where (i), (ii), (iii) and (iv) add up to 100%; optionally in the presence of a liquid medium.
 9. A process according to claim 1 where isocyanate-terminated compound B is at least one polyisocyanate.
 10. A process according to claim 1 where isocyanate-terminated compound B is at least one isocyanate-terminated prepolymer B.
 11. A process according to claim 10 where isocyanate-terminated prepolymer B is obtained by reacting components comprising: (i) 8 to 45 wt % of at least one polyisocyanate; (ii) 0 to 5 wt % of at least one isocyanate-reactive polyol with a weight-average molecular weight in the range of from 50 to 200 g/mol; (iii) 40 to 92 wt % of at least one isocyanate-reactive polyol with a weight-average molecular weight in the range of from 201 to 20,000 g/mol; and (iv) 0 to 3 wt % of an isocyanate-reactive polyol not comprised by (ii) or (iii); where (i), (ii), (iii) and (iv) add up to 100%; optionally in the presence of a liquid medium.
 12. A process according to any claim 8 where the liquid medium comprises <20 wt % of water.
 13. A process according to claim 1 where the solvent borne polyurethane composition has a viscosity <5,000 mPa-s at any solids content in the range of from 45 to 75 wt % in a solvent comprising >70 wt % of at least one solvent having an evaporation rate of >1.0.
 14. An ink comprising a solvent borne polyurethane composition obtained by a process according to claim 1, a colorant and optionally an additional organic solvent.
 15. An ink comprising a solvent borne polyurethane composition, a colorant and optionally an additional organic solvent, wherein the solvent borne polyurethane composition is obtained by a process comprising the following steps: 1) preparing an isocyanate-terminated prepolymer A; 2a) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound; 2b) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 2a) and 2b) together are in the range of from 0.1 to 1.8 stoichiometric equivalents; 3) optionally introducing an isocyanate-terminated compound B; 4a) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.0 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound; 4b) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and the isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.2 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 4a) and 4b) together are in the range of from 0 to 1.2 stoichiometric equivalents; where the process comprises at least one step with an active-hydrogen containing chain-extending compound and at least one step with an active-hydrogen containing chain-terminating compound.
 16. An ink according to claim 14 having a viscosity in the range of from 10 to 100 seconds when measured using a Ford Cup 4 at 20° C.
 17. A method for printing an image on a substrate comprising applying an ink according to claim
 14. 18. A flexographic printing process for printing an image on a substrate comprising applying to the substrate an ink according to claim
 14. 19. A gravure printing process for printing an image on a substrate comprising applying to the substrate an ink according to claim
 14. 20. A laminate comprising a substrate printed with an ink according to claim
 14. 21. A process for making an ink comprising combining a colourant, a solvent borne polyurethane, and optionally an additional organic solvent, wherein the solvent borne polyurethane is obtained by the steps of: 1) preparing an isocyanate-terminated prepolymer A; 2a) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound; 2b) reacting the isocyanate groups of isocyanate-terminated prepolymer A with 0 to 0.95 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 2a) and 2b) together are in the range of from 0.1 to 1.8 stoichiometric equivalents; 3) optionally introducing an isocyanate-terminated compound B; 4a) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.0 stoichiometric equivalents of at least one active-hydrogen containing chain-terminating compound; 4b) reacting the isocyanate groups of the isocyanate-terminated prepolymer obtained in steps 2a) and 2b) and the isocyanate groups of the isocyanate-terminated compound B introduced in step 3) with 0 to 1.2 stoichiometric equivalents of at least one active-hydrogen containing chain-extending compound; where steps 4a) and 4b) together are in the range of from 0 to 1.2 stoichiometric equivalents; where the process comprises at least one step with an active-hydrogen containing chain-extending compound and at least one step with an active-hydrogen containing chain-terminating compound.
 22. A process for making an ink according to claim 21, wherein the ink has a viscosity in the range of from 10 to 100 seconds when measured using a Ford Cup 4 at 20° C.
 23. A method for printing an image on a substrate comprising applying an ink according to claim
 21. 24. A flexographic printing process for printing an image on a substrate comprising applying to the substrate an ink according to claim
 21. 25. A gravure printing process for printing an image on a substrate comprising applying to the substrate an ink according to claim
 21. 